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Photo and Thermal Degradation of a CationicSuperabsorbent PolymerNeelesh Bharti Shukla a , Rajesh Kumar Bhagat a & Giridhar Madras aa Department of Chemical Engineering , Indian Institute of Science , Bangalore , IndiaPublished online: 04 Jan 2013.

To cite this article: Neelesh Bharti Shukla , Rajesh Kumar Bhagat & Giridhar Madras (2013) Photo and ThermalDegradation of a Cationic Superabsorbent Polymer, Polymer-Plastics Technology and Engineering, 52:1, 58-65, DOI:10.1080/03602559.2012.719171

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Photo and Thermal Degradation of a CationicSuperabsorbent Polymer

Neelesh Bharti Shukla, Rajesh Kumar Bhagat, and Giridhar MadrasDepartment of Chemical Engineering, Indian Institute of Science, Bangalore, India

A cationic superabsorbent polymer (SAP) was synthesized bycarrying out the polymerization of [2-(methacryloyloxy)ethyl] tri-methyl ammonium chloride) with N,N0-methylenebisacrylamide asthe cross-linking agent. The SAP was subjected to degradation indry and the equilibrium swollen state by thermo gravimetric analysisand exposure to ultraviolet radiation, respectively. The photodegra-dation was monitored by measuring changes in the swelling capacityand the dry weight of the SAP. The thermal degradation of the SAPoccurred in three stages after the initial removal of moisture and theactivation energies of the decomposition were determined.

Keywords Photodegradation; Superabsorbent; Swelling; Ther-mal degradation

INTRODUCTION

Three-dimensional cross-linked networks of hydrophilicpolymers that are able to absorb and retain liquids morethan 100 times of their own weight are known as superab-sorbent polymers (SAPs)[1]. The SAPs contain ionic repeatunits, and could classified be as anionic, cationic, ampho-teric, or zwitterionic[2]. They find applications in personalhygiene products[2], food packaging[2], wound healingdevices[3] and water treatment[4]. Although, most of theSAPs commercially produced are anionic (based on acrylicacid and its derivatives)[2], a wide variety of other superab-sorbent polymers have been synthesized[5–9]. The cationicmonomers have also been polymerized, either indepen-dently[10] or with anionic SAPs to obtain amphotericSAPs[11–15].

The synthesis routes of the monomer-based SAPs arewell established, but there are only a few studies reportingtheir degradation[16–17].The SAPs constitute a significantpart of domestic waste, making the degradation studiesof the synthetic SAPs important. Li and Cui[16] subjectedacrylic acid and acrylamide-based SAP to ultraviolet-induced decomposition and quantified the degradation bymeasuring the weight of the SAP at the end of the degra-dation. Their study, although the first report of its kind,

did not provide information about the changes in theswelling capacity of the SAPs during the degradation. Inour previous study, we had adopted an improved techniquefor the photodegradation of poly (acrylic acid-co-sodiumacrylate-co-acrylamide) SAPs[17].

The changes in the swelling capacity as well as theresidual weights of the SAPs were measured during thedegradation and a possible mechanism of the degradationof SAPs was proposed. The SAPs were found to undergodegradation in two stages. In the first stage, the networkbecame sparse and the swelling capacity increased but theresidual weight fraction was constant. The rupture of thethree-dimensional network took place in the second stage,and thus both the swelling capacity and the residualweights reduced drastically.

In the present study, we report the photodegradation ofa cationic SAP by the earlier developed method[18]. Here weextend the study to investigate the effect of cross-linkdensity on the photodegradation of the SAP. The cationicSAP poly([2-(methacryloyloxy)ethyl]trimethyl ammoniumchloride) (PMALETMAC) was synthesized by solutionpolymerization. The content of cross-linker N,N0-methylenebisacrylamide (MBA) was varied to obtain SAPsof different equilibrium swelling capacities. The photo-degradation was monitored by measuring the swellingcapacity and the residual weight of the SAP during thephotodegradation. As the disposed SAPs could be in equi-librium swollen as well as in the dry state, the degradationwas also carried out by thermo gravimetric analysis (TGA)in the dry state and the activation energy was calculated.

EXPERIMENTAL

Materials

Cationic monomer [2-(methacryloyloxy)ethyl]trimethylammonium chloride (MALETMAC) and tetra-functionalcross-linker N,N0-methylenebisacrylamide (MBA) werepurchased from Sigma-Aldrich (USA) and S.D. Fine-Chem Ltd. (Mumbai, India), respectively. Initiatorammonium persulfate (APS) and accelerator N,N,N0,N0-tetramethylethylenediamine (TEMED) were procuredfrom S.D. Fine-Chem Ltd. (Mumbai, India) and Fluka,

Address correspondence to Giridhar Madras, Department ofChemical Engineering, Indian Institute of Science, Bangalore560012, India. E-mail: giridhar@chemeng.iisc.ernet.in

Polymer-Plastics Technology and Engineering, 52: 58–65, 2013

Copyright # Taylor & Francis Group, LLC

ISSN: 0360-2559 print=1525-6111 online

DOI: 10.1080/03602559.2012.719171

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respectively. Milli-Q deionized (DI) water was used for allthe experiments.

Synthesis of the Cationic SAP

Cationic monomer MALETMAC (5.66ml, 80wt% sol-ution) was taken in a 50-ml beaker and the requiredamount of cross-linker MBA was added it. Monomerand cross-linker were allowed to mix by stirring undernitrogen flow. Initiator APS (0.0275 g, 0.5mol % of mono-mer) was added to the monomer-cross-linker mixture andallowed to dissolve under nitrogen flow. The acceleratorTEMED (2.8ml, 0.5wt%) was added and the reactionmixture was allowed to polymerize for 24 h at room tem-perature. The obtained polymer was cut into pieces andrepeatedly washed with DI water and then dried in ahot-air oven till constant weight was achieved. The cross-link density of the cationic SAP was varied by varyingthe content of MBA.

Fourier Transform-Infrared (FTIR) Spectroscopy

Fourier transform infrared (FTIR) spectrum of thePMALETMAC was recorded on a Perkin Elmer Spectrum1000 FT-IR Spectrometer (USA). The dry SAP was pow-dered and mixed with KBr to form thin discs. The FTIRspectrum was obtained by scanning in transmittance modein the range of 4000–500 cm�1.

Determination of Equilibrium Swelling Capacity

The swelling capacity is defined as: S¼ (Ws�Wd)=Wd,where Wd and Wf are the initial dry weight and theweight of the swollen SAP, respectively. The swellingcapacity of the cationic SAP was determined gravimetri-cally. The 0.10 g (�0.0050 g) of the dry SAP was kept ina plastic basket and immersed in glass beaker containingDI water (500ml). The baskets were taken out at differenttimes, and were wiped with tissue paper to remove theexcess water. The swollen samples were weighed on aweighing balance (Sartorius BP121 S) and were returnedto the respective beakers for further swelling. Sixsamples were taken for each experiment, and the reporteddata points represent the averages of the six data points.The error in swelling capacity was found to be lesser than�3%.

Photodegradation of the SAP

Photodegradation of the SAP was carried out in theequilibrium swollen state. 0.10 g (�0.0050 g) of the drySAP was swollen to equilibrium in DI water for 6 h. Thephotodegradation was carried out using a 125W high-pressure mercury vapor (HPMV) lamp (Philips, India) asan ultraviolet (UV) radiation source. The lamp was placedinside a jacketed quartz tube (3.4 cm i.d., 4 cm o.d., and20 cm length), and a constant temperature was maintainedby water circulation. The intensity of the UV radiation

emitted by HPMV lamp was measured using a UVlight meter (UV-340, Lutron) and was found to be590 mW cm�2.

The glass beakers containing equilibrium swollen SAPswere arranged concentrically around the lamp (Fig. 1)and exposed to UV radiation for a certain period of time.The degradation of SAPs was monitored by measuringthe changes in the swelling capacity and the residualweight. The samples degraded by exposure to UV radiationwere transferred to perforated cups made of aluminum foil.The excess water was drained and the swelling capacity wasdetermined gravimetrically. The aluminum foil cups con-taining swollen SAPs were kept in hot-air oven for dryingand then weighed to determine the residual weight of theSAP.

Thermal Degradation of the SAP

The thermal degradation of the SAP was carried out ona Pyris TG=DTA system (PerkinElmer, USA). About 7mgof the dry SAP was subjected to thermal degradation in aninert atmosphere under the nitrogen flow (150 cm3min�1).The SAP was heated from 30 to 500�C at constant heatingrates of 5, 7, 10, 15 and 20Kmin�1.

RESULTS AND DISCUSSION

Fourier Transform Infrared Spectroscopy

The FTIR spectrum of cationic SAP PMALETMAC isshown in Figure 2. The characteristic peak at 1732.8 cm�1

is assigned to the stretching vibrations of the carbonylgroups. The peaks due to the bending and stretchingvibrations of quaternary ammonium group appear at1489.1 cm�1 and 955.5 cm�1, respectively[18,19].

FIG. 1. The schematic of photodegradation setup.

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Swelling of the SAP

The PMALETMAC superabsorbent contains cationicpendant quaternary ammonium groups (fixed charges)and mobile charges chloride ions as counterion in its repeatunits. The presence of the cationic charges generates repul-sion forces tending to expand the network. The concen-tration difference in the mobile ions between the polymerand the swelling medium causes the osmotic pressure dif-ference which drives the water inside the network. As aresult, the SAP swells till the expansion of the network isprevented by the elastic retractive forces[20,21].

Figure 3 shows the variation in the swelling capacity ofthe SAP (0.5wt% MBA) with time. The SAP absorbedwater at a faster rate in the beginning and the amount ofwater absorbed became constant after a certain period oftime as it achieved its equilibrium swelling capacity. Theswelling of the SAP followed first order kinetics: S¼Seq[1�exp(�kst)], where S and Seq are the swelling capacityat time t and at equilibrium, and ks is the rate constant ofswelling. The Seq and ks of the SAP were found to be

141.8 g=g and 4.69 h�1, respectively. Similar first-orderswelling kinetics has been observed for the poly(acrylicacid-co-sodium acrylate-co-acrylamide)[21], hydrogels ofacrylamide[22], carboxymethyl cellulose-g-poly(acrylamide-co-2-acrylamido-2-methylpropan sulfonic acid)[23] andstarch-based superabsorbents[24].

Figure 4 shows the variation in the equilibrium swellingcapacity of the PMALETMAC superabsorbent with theconcentration of MBA. The swelling capacity of the SAPdecreased with increasing MBA concentration. The increa-sing MBA content (0.025 to 2.5mol % of the monomer)gave rise to a denser three-dimensional network resultingin the lowering of the equilibrium swelling capacity of theSAP.

Photodegradation of the SAP

The swelling capacity is very low above 1.0mol% MBAand the network is too fragile below 0.25mol % MBA.Therefore only SAPs (0.25, 0.5 and 1mol% MBA) weresubjected to photodegradation in the equilibrium swollenstate. The variation in the swelling capacity and the dryweight of the SAPs were monitored during degradation.The SAPs subjected to UV exposure were dried andweighed to determine the change in weight during thedegradation. Figures 5(a) and 5(b) show the variation inswelling capacity and the dry weight with UV exposuretime for the SAPs of various cross-link densities.Figures 5(a) and 5(b) show the variation in S=S0 andW=W0 for the photodegradation of PMALETMAC with0.25, 0.5 and 1.0mol % MBA contents, respectively.

S0 and S are the swelling capacities at time t¼ 0 and atdegradation time t, respectively. Similarly, W0 and W rep-resent the weights of the dry SAP at time t¼ 0 and atdegradation time t, respectively. The decrease in the cross-linker content resulted in higher extent of degradation. TheSmax to S0 ratios decreased with decreasing cross-linker

FIG. 2. Fourier transform infrared (FTIR) spectrum of PMALETMAC.

FIG. 3. Increment in swelling capacity of PMALETMAC with time.

FIG. 4. Variation in the equilibrium swelling capacity of PMALET

MAC with MBA content.

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content and followed the order: 1.0> 0.5>0.25mol% MBA (Table 1).

The photodegradation of all the SAPs took place in twostages. In the first stage, the swelling capacity increased andreacheda maximum value, Smax, and decreased during thesecond stage of degradation (Fig. 5(a)). During the firststage, the scission of the cross-links took place and thecross-link density of the network became sparse, allowingmore water to flow leading to an increment in the swellingcapacity. Further exposure to UV radiation led to the rup-ture of the network, causing reduction in swelling capacity.

The dry weights of the SAPs decreased throughout thedegradation (Fig. 5(b)). Significant reduction in the weight

was not observed during the first stage as only thecross-links broke without the formation of water solublepolymer. The cross-linked network ruptured in the secondstage, leading to the formation of the water soluble poly-mers causing significant reduction in the weight of theSAP. The observation of the dry weights along with theswelling capacity supports the proposed degradationphenomenon. These results are in accordance with thephotodegradation of poly(acrylic acid-co-sodium acrylate-co-acrylamide) SAPs[17].

The increment of the swelling capacity, as mentionedearlier, occurs due to the lowering of the cross-link density.In a SAP with low cross-link density, the rupture occursfaster and the increment in swelling capacity is low. Thisis confirmed by the comparison of the reduction in theweight (Fig. 5(b)). The reduction in the weight takes placeat a faster rate and to a higher extent for the SAPs withlower cross-link density (Table 1).

Thermal Degradation of the SAP

The thermal stability of the hydro gels was investigatedby thermo gravimetric analysis (TGA). The thermal degra-dation of the SAP with 0.5mol% MBA carried out at con-stant heating rates: 5, 7, 10, 15 and 20�C=min and SAPs(0.025, 0.050, 0.25, 0.50, 1.5 and 2.5mol% MBA) were car-ried out at constant heating rate 10�C=min, in the nitrogenatmosphere. The weight loss profiles of the SAPs are shownin Figures 6(a) and 7(a). The SAP lost about 12–17%weight from 30–225�C. This weight loss was due to theevaporation of the moisture in the SAPs. Goel et al. havereported TGA of MALETMAC, MALETMAC-g-cottonand cotton. They observed a weight loss of about 30%below 250�C for MALETMAC and attributed it to thehygroscopic nature of the polymer[25].

Above 225�C the SAPs with 0.025 and 0.050mol %MBA degraded in two stages where as all other SAPsdegraded in three stages (Table 2). The three stage degra-dation is evident from the DTG maxima observed inFigures 6(b) and 7(b). For the degradation of SAP with0.5% MBA, during the first and second stages, 225 to340�C, the W=W0 of the SAP decreased from about 0.85to 0.35, indicating about 50% loss in weight. In the thirdstage of decomposition, 340–450�C, the W=W0 reducedfrom 0.35 to 0.05. The TGA of MALETMAC-AA resin[26]

also exhibited about 96% weight loss up to 500�C.

FIG. 5. Variation in the (a) swelling capacity, and (b) weight during the

photodegradation of PMALETMAC with different MBA content. (Color

figure available online.)

TABLE 1Effect of the MBA content on the photodegradation of PMALETMAC

MBA (mol %) Seq (g=g) Smax=S0 S=S0 at 180min W=W0 at 180min

1.0 66.5 1.29 0.246 0.480.5 141.8 1.18 0.148 0.230.25 241.2 1.05 0.04 0.06

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The volatile products formed during the first stage ofdegradation are due to the removal of quaternaryammonium groups, the second stage arises due to highercross-link density and kinetic nonequivalence of reactivegroups. The polymer decomposed completely by randomscission and charring in the third stage of degradation[25].

When the cross-link density was very less (0.025 and0.05mol% MBA), only two stages of degradation wereobserved and the effect of cross-link density was notobserved.

The multiple heating rate methods, Kissinger andOzawa, and the single heating rate Friedman method wereused to determine the activation energies. All the methodsindicated three stages of the degradation of the polymer,and the kinetic parameters across the methods matchedclosely.

Determination of Activation Energy by the FriedmanMethod

The activation energy is calculated from the TGA dataat a single heating rate by Friedman method. It involvesthe comparison of mass loss rates for a fractional mass lossa and the expression is given as:

lndadt

� �¼ ln Zþ ln fðaÞ � Ea

RTð1Þ

The function is assumed to be f(a)¼ (1� a)n and the slope oflinear plot of ln (da=dt) vs. 1=T is used to obtain the appar-ent activation energy. The Friedman plot obtained at a heat-ing rate of 10�C=min is shown in Figures 8(a), (b), (c) for thethree stages, respectively. For the degradation of SAPs(0.025 and 0.05mol% MBA), two overlapping straight linesand for SAPs (0.25, 0.50, 1.5 and 2.5mol% MBA), threeoverlapping straight lines were obtained and the 2D water-fall graph (in Origin 6.1 software) with 60% offset was usedto clearly represent the data. The activation energy wasdetermined at a heating rate of 10�C=min and are listed inTable 2.

Determination of Activation Energy by theKissinger Method

The activation energy for the degradation ofPMALETMAC was calculated by the Kissingermethod[17]. It involves the determination of activationenergy at the maximum decomposition rate using thethermo grams at various heating rates. The expressionfor the calculation of activation energy is

d½LnH�d½1=Tm�

¼ �E

Rð2Þ

where H ¼ b=T2m, b is the heating rate (K=min) and Tm(K)

corresponds to the temperature at the maximum rate ofdecomposition. E and R denote the activation energyand the universal gas constant, respectively. The activationenergy of the decomposition of the SAPs was calculatedfrom the slopes of plot of Ln H with the inverse of tem-perature (Fig. 9) and was found to be 132.6, 91.5 and157.5 for the three stages of degradation.

FIG. 6. (a) The weight loss and the (b) DTG for SAPs (with 0.025, 0.05,

0.25, 0.50, 1.5, and 2.5mol % MBA). (Color figure available online.)

TABLE 2Activation energies of SAPs of different MBA

content of PMALETMAC at a heating rate 10�C=minfor the three stages

Activation energies (kJ=mol)

MBA (mol %) First stage Second stage Third stage

0.025 103 – 1120.05 112 – 1520.25 141 94 1430.5 124 87 1451.5 107 209 1402.5 104 212 136

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Determination of Activation Energy by the Ozawa Method

The Ozawa technique is also a multiple heating ratemethod to determine the activation energyas a functionof conversion and the expression is given as:

log

Z a

0

dafðaÞ

� �¼ log

ZEa

R

� ��logb� 2:315� 0:4567

Ea

RT

� �

ð3Þ

where a is conversion at time t and f(a) represents somefunction of conversion. a is defined as: a¼ (M0�Mt)=(M0�Mf), where M0 and Mf are the initial and finalweights, and Mt is weight at any time. Ea and Z representthe apparent activation energy and the pre-exponential fac-tor. Ozawa’s technique assumes temperature independenceof Z, (1�a), n, and Ea, and conversion independence Zand Ea.

Figures 10(a) and 10(b) show the Ozawa plots obtainedat different conversions. The activation energy is obtainedfrom the slope of log b vs. 1=T, where the b is the heatingrate. The activation energies obtained at 20, 30, 40, 50, 60,70, 80, and 90% conversion are 127.4, 128.5, 125.8, 107.7,106.8, 138.4, 160.2, and 163.8 kJ=mol, respectively.

FIG. 7. (a) The weight loss and the (b) DTG profiles of PMALETMAC

(0.5mol % MBA) at heating rates of 5,7, 10, 15 and 20�C=min. (Color

figure available online.)

FIG. 8. Friedman plot for the degradation of PMALETMAC (0.025,

0.050, 0.25, 0.5, 1.5 and 2.5mol% MBA) at a heating rate of 10�C=min,

for (a) first (b) second and (c) third degradation steps.

FIG. 9. Kissinger plot for the thermal degradation of PMALETMAC

(0.5mol % MBA). (Color figure available online.)

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CONCLUSIONS

A cationic hydrogel PMALETMAC was synthesized bysolution polymerization using MBA as cross-linker. Theequilibrium swelling capacity of the SAPs decreased withincreasing MBA content (0.025–2.5mol%). The degradati-on of the SAP was carried out in the dry and equilibriumswollen state by thermogravimetric analysis and exposureto UV radiation, respectively. The photodegradation ofthe SAP took place in two stages. The equilibrium swellingcapacity increased and reached to a maximum as thecross-links broke and network became sparse. The swellingcapacity decreased in the second stage. The dry weight ofthe SAP decreased throughout the photodegradation.

The initial reduction in the weight was not significant asonly the cross-links were broken, but subsequently thenetwork ruptured, producing a water-soluble polymer,resulting in significant weight loss. The extent of the photo-degradation increased with decreasing cross-linker contentas the cross-link density decreased. For high cross-linkedSAPs, the thermal degradation occurred in three stages afterthe initial loss of moisture and for very low cross-linked

SAPs, it occurs in two stages. The activation energies forall the stages were calculated by the Kissinger, Ozawa, andFriedman methods and were found to be in reasonableagreement.

ACKNOWLEDGMENTS

The authors thank the department of science andtechnology for financial support.

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